the trajectory of the campaign, his selection of a running mate, paul ryan, got a small bounce. the convention speech didn't get any bounce, and now the first debate. this is like the first night on broadway. there will be more voters tuning in, they will be looking at his performance and if he comes across as somebody who is credible, who is up to the task, he might get another bounce out of it. >> aren't undecided voters the least likely to tune into the debates? gallup looked back at records and found only two examples where debates really changed the course. that was 1960 and 2000. >> they are important moments for these candidates but you're exactly right. it's so hard to change anything at this point. why. these two guys are not unpainted canvases. barack obama, his canvas is pretty much filled in. we know him. we have seen him for four years. as president. mitt romney has been through a long and grueling campaign. there is very little white space left on his canvas. so if they're really going to do something different that changes how they're understood, it has to be something very big.